The Forestry Commission is warning people to be on the look out for toxic caterpillars that can cause life-threatening asthma attacks, vomiting and skin rashes.
The Oak Processionary Moth was first identified in London in 2006 and has since spread to the surrounding counties.
Last year, the tree pest was spotted in Cambridgeshire, Hertfordshire, Essex and Lincolnshire.
The caterpillars and their nests contain hairs which can cause itchy rashes, eye and throat irritations, and should not be touched under any circumstances at any time.
The greatest risk period is May to July when the caterpillars emerge and feed before pupating into adult moths.
OPM caterpillars feed on oak leaves and can increase trees’ vulnerability to attack by other pests and diseases, making them less able to withstand adverse weather conditions such as drought and floods.
Large populations can strip whole oak trees bare.
A government programme is in place to limit the spread from areas where they are present.
Andrew Hall, Forestry Commission Operations Manager, said:
How to identify OPM caterpillarsNests are typically dome or teardrop-shaped, averaging the size of a tennis ball. They are white when fresh, but soon become discoloured and brown. The caterpillars have black heads and bodies covered in long white hairs which contain proteins which can cause itchy rashes, eye, and throat irritations. They can also occasionally cause breathing difficulties in people and pets, so should not be touched under any circumstances.For more information on how to identify OPM, visit www.forestresearch.gov.uk/opm